A cholesteric liquid crystal is a bistable material which has an unstable phase called a homeotropic phase and stable phases called focal conic and planar phases. A voltage application by various methods can cause a transition between the respective phases. A cholesteric liquid crystal which is in a focal conic state or in a planar state is also in a stable state after a voltage application to liquid crystals is stopped. Therefore, such a cholesteric liquid crystal is used as a memory liquid crystal. Such a characteristic of cholesteric liquid crystal causes active development of cholesteric liquid crystal display devices.
A cholesteric liquid crystal display device includes a liquid crystal panel having a simple matrix electrode structure, a common driver for driving a common electrode, and a segment driver for driving a segment electrode. A method for driving such a cholesteric liquid crystal display device is roughly classified into a conventional drive method and a DDS (Dynamic Drive Scheme) drive method (see Patent Literature 1).
The conventional drive method can be simply carried out by use of a driver of a general-purpose STN (Super Twisted Nematic) liquid crystal display device. In order to obtain a planar state by the conventional drive method, a voltage of not less than a given voltage Vp is applied for a given time. In order to obtain a focal conic state by the conventional drive method, a given voltage Vfc is applied for a given time. In order to obtain a uniform display state, a full screen is uniformly reset to be in a planar state and further to be in a focal conic state, so that writing is carried out.
The DDS drive method can be carried out in a shorter voltage application time than the conventional drive method. The DDS drive method includes the following three steps: (i) resetting a full screen to be in a planar state at one time, (ii) applying “a voltage pulse for determining a final state” for a short time”; and (iii) applying a retention voltage called a non-selection voltage so as to cause a final state.
Patent Literature 2 discloses a method in which according to the DDS drive method, a non-selection period of time is optimized, so as to remove display unevenness. Patent Literature 3 discloses a method in which according to the DDS drive method, an entire screen is in a non-selection state after a non-selection period, so as to close a contrast difference between a first line and a final line. Patent Literature 4 discloses a method in which according to the conventional drive method, a segment mode and a common mode of a driver are switched, so as to simply carry out a reset operation on a full screen.